Lifetime instabilities in gallium doped monocrystalline PERC silicon solar cells

Gallium doped silicon is an industrially viable alternative to boron doped silicon for photovoltaics, and is assumed to be immune from light-induced degradation. We have studied light soaking for >1000 h of industrially fabricated passivated emitter and rear cell (PERC) devices formed from monocrystalline gallium and boron doped substrates, with cell properties monitored using a non-contact photoluminescence imaging proxy method. As-fabricated stabilised boron doped cells did not degrade or underwent a slight improvement, whereas as-fabricated gallium doped cells which had not been intentionally stabilised experienced a slight (~5%) deterioration which then recovered. When PERC devices were subjected to a 200–300 °C dark anneal before light soaking, significant differences in the cell degradation signatures were observed. Degradation characteristic of light and elevated temperature induced degradation (LeTID) was observed for boron and gallium PERC solar cells, with the onset of degradation taking longer, and the severity being less, with gallium. Investigation of stripped gallium PERC devices with room temperature surface passivation revealed bulk lifetime degradation correlates with the cell-level degradation. When the cells were stripped and passivated with aluminium oxide, a complete change in the degradation behaviour was observed, with no degradation occurring in the gallium case and boron-oxygen-like degradation observed for boron. This indicates that dielectric passivation is not suitable for lifetime degradation diagnosis in stripped cells. Gallium retains advantages over boron doping with stabilisation processes not generally required, but manufacturers need to be aware of possible low-temperature lifetime instabilities when developing future fabrication processes, such as for passivated contact structures.